This invention teaches novel structure and methods for achieving efficient collection and conveyance of power from photovoltaic power generating devices.
Photovoltaic cells have developed according to two distinct methods. The initial operational cells employed a matrix of single crystal silicon appropriately doped to produce a planar p-n junction. An intrinsic electric field established at the p-n junction produces a voltage by directing solar photon produced holes and free electrons in opposite directions. Despite good conversion efficiencies and long-term reliability, widespread energy collection using single-crystal silicon cells is thwarted by the high cost of single crystal silicon material and interconnection processing.
A second approach to produce photovoltaic cells is by depositing thin photovoltaic semiconductor films on a supporting substrate. Material requirements are minimized and technologies can be proposed for mass production. Thin film photovoltaic cells employing amorphous silicon, cadmium telluride, copper indium gallium diselenide, dye sensitized solar cells (DSSC), printed silicon inks and the like have received increasing attention in recent years.
Despite significant improvements in individual cell conversion efficiencies for both single crystal and thin film approaches, photovoltaic energy collection has been generally restricted to applications having low power requirements. One factor characteristic of many optoelectric devices and photovoltaic cells in particular is that electrical energy is produced over a relatively expansive surface area. Thus a challenge to implementing bulk power systems is the problem of economically collecting the photogenerated power from an expansive surface. In particular, photovoltaic cells can be described as high current, low voltage devices. Typically individual cell voltage is less than about two volts, and often less than 0.6 volt. The current component is a substantial characteristic of the power generated. Efficient power collection from expansive photovoltaic cell surfaces must minimize resistive losses associated with the high current characteristic. In the specific case of most photovoltaic cells, the upper surface is normally formed by a transparent conductive oxide (TCO). However, these TCO layers are relatively resistive compared to pure metals and have a surface resistivity on the order of 10 to 100 ohms per square. Thus the conductive surface itself is limited in its ability to collect and transport current and efforts must be made to minimize resistive losses in transport of current through the TCO layer. This problem increases in severity as individual cell sizes increase. One solution is to simply reduce individual cell size (and thus accumulated current from an individual cell) to a point where the transparent conductive oxide alone can handle the current. Where larger individual cell sizes are the norm, it is common practice to augment the transparent conductive oxide with a current collector structure comprising a pattern of highly conductive traces extending over substantially the entire surface from which current is to be collected. Often the structure is in the form of a grid or lattice pattern. The current collector structure reduces the distance that current must be transported by the transparent conductive oxide before it reaches a highly conductive conveyance path off the surface. Thus the current collector structure collects current from a surface having relatively low surface conductivity. Many current collector structures or grids are conventionally prepared by first applying metal wires, fused silver filled pastes or silver filled ink traces to the cell surface and then covering the surface with a sealing material in a subsequent operation. These highly conductive traces may lead to a collection buss such as a copper foil strip which also functions as a tab extending to the back electrode of an adjacent cell. The wire approach requires positioning and fixing of multiple fine fragile wires which makes mass production difficult and expensive. Silver pastes are expensive and require high fusion temperatures which not all photovoltaic semiconductors can tolerate. A silver filled ink, as compared to a fuseable paste, is simply dried or cured at mild temperatures which do not adversely affect the cell. However, this ink approach requires the use of relatively expensive inks because of the high loading of finely divided silver particles. In addition, batch printing on the individual cells is laborious and expensive. Finally, the silver filled ink is relatively resistive compared to a fuseable silver paste or metal wire. Typical silver filled inks have intrinsic resistivities in the range 0.00002 to 0.01 ohm-cm.
Thus there remains a need for improved materials and structure for collecting the current from the top light incident surface of photovoltaic cells.
Normally one envisions a photovoltaic power collection device much larger than the size of an individual cell. Therefore, an arrangement must be supplied to collect power from multiple cells. This is normally accomplished by interconnecting multiple cells in series. In this way, voltage is stepped through each cell while current and associated resistive losses are minimized. Such interconnected multi-cell arrangements are commonly referred to as “modules” or “arrays”. However, it is readily recognized that making effective, durable series connections among multiple small cells can be laborious, difficult and expensive. Regarding traditional crystalline silicon cells, the individual cells are normally discrete and comprise rigid wafers approximately 200 micrometers thick and approximately 230 square centimeters in area. A common way to convert multiple such cells into modules is to use a conventional “string and tab” arrangement. In this process multiple discrete cells are arranged in “strings” and the topside current collector electrodes of cells are connected to backside electrodes of adjacent cells using “tabs” or ribbons of conductive material. The cell connections often involve tedious manual operations such as soldering and handling of multiple interconnected cells. Next, unwieldy flexible leads from the terminal cells must be directed and secured in position for outside connections, again a tedious operation. Finally, weight and assembly concerns limit the ultimate size of the module. These limitations impede adoption of the modules for large scale power generation.
In order to approach economical mass production of modules of series connected individual cells, a number of factors must be considered in addition to the type of photovoltaic materials chosen. These include the substrate employed and the process envisioned. Since thin films can be deposited over expansive areas, thin film technologies offer additional opportunities for mass production of interconnected modules compared to inherently small, discrete single crystal silicon cells. Thus a number of U.S. patents have issued proposing designs and processes to achieve series interconnections among thin film photovoltaic cells. Many of these technologies comprise deposition of photovoltaic thin films on glass substrates followed by scribing to form smaller area individual cells. Multiple steps then follow to electrically connect the individual cells in series while maintaining the original common glass substrate. These “common” substrate approaches have come to be known as “monolithic integration”. Examples of these proposed processes are presented in U.S. Pat. Nos. 4,443,651, 4,724,011, and 4,769,086 to Swartz, Turner et al. and Tanner et al. respectively. While expanding the opportunities for mass production of interconnected cell modules compared with inherently discrete approaches for crystal silicon cells, monolithic integration employing common glass substrates must inherently be performed on an individual batch basis. In addition, many monolithic approaches are not compatible with the use of a current collector grid and therefore cell sizes (in the direction of current flow) are constrained. Typically, cell widths for monolithic integration between 05 cm. and 1.0 cm. are taught in the art. However, as cell widths decrease, the width of the area between individual cells (interconnect area) should also decrease so that the relative portion of inactive surface of the interconnect area does not become excessive. These small cell widths demand very fine interconnect area widths, which dictate delicate and sensitive techniques to be used to electrically connect the top TCO surface of one cell to the bottom electrode of an adjacent series connected cell. Furthermore, achieving good stable ohmic contact to the TCO cell surface has proven difficult, especially when one employs those sensitive techniques available when using the TCO only as the top collector electrode.
More recently, developers have explored depositing wide area films using continuous roll-to-roll processing. This technology generally involves depositing thin films of photovoltaic material onto a continuously moving web. However, a challenge still remains regarding subdividing the expansive films into individual cells followed by interconnecting into a series connected array. For example, U.S. Pat. No. 4,965,655 to Grimmer et. al. and U.S. Pat. No. 4,697,041 to Okamiwa teach processes requiring expensive laser scribing and interconnections achieved with laser heat staking. In addition, these two references teach a substrate of thin vacuum deposited metal on films of relatively expensive polymers. The electrical resistance of thin vacuum metallized layers may significantly limit the active area of the individual interconnected cells.
It has become well known in the art that the efficiencies of certain promising thin film photovoltaic junctions can be substantially increased by high temperature treatments. These treatments involve temperatures at which even the most heat resistant plastics suffer rapid deterioration. Use of a metal foil as a substrate allows high temperature treatments and continuous roll-to-roll processing. However, the subsequent conversion to an interconnected module of multiple cells has proven difficult, in part because the metal foil substrate is electrically conducting.
U.S. Pat. No. 4,746,618 to Nath et al. teaches a design and process to achieve interconnected arrays using roll-to-roll processing of a metal web substrate such as stainless steel. The process includes multiple operations of cutting, selective deposition, and riveting. These operations add considerably to the final interconnected array cost.
U.S. Pat. No. 5,385,848 to Grimmer teaches roll-to-roll methods to achieve integrated series connections of adjacent thin film photovoltaic cells supported on an electrically conductive metal substrate. The process includes mechanical or chemical etch removal of a portion of the photovoltaic semiconductor and transparent top electrode to expose a portion of the electrically conductive metal substrate. The exposed metal serves as a contact area for interconnecting adjacent cells. These material removal techniques are troublesome for a number of reasons. First, many of the chemical elements involved in the best photovoltaic semiconductors are expensive and environmentally unfriendly. This removal subsequent to controlled deposition involves containment, dust and dirt collection and disposal, and possible cell contamination. This is not only wasteful but considerably adds to expense. Secondly, the removal processes are difficult to control dimensionally. Thus a significant amount of the valuable photovoltaic semiconductor is lost to the removal process. Ultimate module efficiencies are further compromised in that the spacing between adjacent cells grows, thereby reducing the effective active collector area for a given module area.
Thus there remains a need for acceptable mass manufacturing processes and articles to achieve effective integrated interconnections among photovoltaic cells.
A further issue that has impeded adoption of photovoltaic technology, especially for bulk power collection in the form of solar farms, involves installation of multiple modules over expansive regions of surface. Traditionally, modules have been mounted individually on supporting mounts, normally at an incline to horizontal appropriate to the latitude of the site. Conducting leads from each module are then physically coupled with leads from an adjacent module in order to interconnect multiple modules. This arrangement results in a string of modules each of which is coupled to an adjacent module. At one end of the string, the power is transferred from the end module to be conveyed to a separate site for further treatment such as voltage adjustment. This arrangement avoids having to run conductive cabling from each individual module to the separate treatment site.
The traditional solar farm installation described in the above paragraph has some drawbacks. First, traditional modules are limited in size due to weight and manufacturing constraints. This fact increases the number of individual modules required to cover a desired surface area. Next, the module itself comprises a string of individual cells. In the conventional module lead conductors in the form of flexible wires or ribbons are attached to an electrode on the two cells positioned at each end of the string in order to convey the power from the module. After mounting the individual modules on their support at the installation site, the respective leads from adjacent modules must be connected in order to couple adjacent modules, and the connection must be protected to avoid environmental deterioration or separation. These are intrinsically tedious manual operations. Finally, since the module leads and cell interconnections are not of high current carrying capacity, the adjacent cells are normally connected in series arrangement. Thus voltage builds up to high levels even at relatively short strings of modules. While not an overriding problem security and insulation must be appropriate to eliminate a shock hazard.
Thus there remains a need for improved module form factors and complimentary installation structure to reduce the cost and complexity of achieving large area “utility” scale photovoltaic installations.
In a somewhat removed segment of technology, a number of electrically conductive fillers have been used to produce electrically conductive polymeric materials. This technology generally involves mixing of a conductive filler such as silver particles with the polymer resin prior to fabrication of the material into its final shape. Conductive fillers may have high aspect ratio structure such as metal fibers, metal flakes or powder, or highly structured carbon blacks, with the choice based on a number of cost/performance considerations. More recently, fine particles of intrinsically conductive polymers have been employed as conductive fillers within a resin binder. Electrically conductive polymers have been used as bulk thermoplastic compositions, or formulated into paints and inks. Their development has been spurred in large part by electromagnetic radiation shielding and static discharge requirements for plastic components used in the electronics industry. Other known applications include resistive heating fibers and battery components and production of conductive patterns and traces. The characterization “electrically conductive polymer” covers a very wide range of intrinsic resistivities depending on the filler, the filler loading and the methods of manufacture of the filler/polymer blend. Resistivities for filled electrically conductive polymers may be as low as 0.00001 ohm-cm. for very heavily filled silver inks, yet may be as high as 10,000 ohm-cm or even more for lightly filled carbon black materials or other “anti-static” materials. “Electrically conductive polymer” has become a broad industry term to characterize all such materials. In addition, it has been reported that recently developed intrinsically conducting polymers (absent conductive filler) may exhibit resistivities comparable to pure metals.
In yet another separate technological segment, coating plastic substrates with metal electrodeposits has been employed to achieve decorative effects on items such as knobs, cosmetic closures, faucets, and automotive trim. The normal conventional process actually combines two primary deposition technologies. The first is to deposit an adherent metal coating using chemical (electroless) deposition to first coat the nonconductive plastic and thereby render its surface highly conductive. This electroless step is then followed by conventional electroplating. ABS (acrylonitrile-butadiene-styrene) plastic dominates as the substrate of choice for most applications because of a blend of mechanical and process properties and ability to be uniformly etched. The overall plating process comprises many steps. First, the plastic substrate is chemically etched to microscopically roughen the surface. This is followed by depositing an initial metal layer by chemical reduction (typically referred to as “electroless plating”). This initial metal layer is normally copper or nickel of thickness typically one-half micrometer. The object is then electroplated with metals such as bright nickel and chromium to achieve the desired thickness and decorative effects. The process is very sensitive to processing variables used to fabricate the plastic substrate, limiting applications to carefully prepared parts and designs. In addition, the many steps employing harsh chemicals make the process intrinsically costly and environmentally difficult. Finally, the sensitivity of ABS plastic to liquid hydrocarbons has prevented certain applications. ABS and other such polymers have been referred to as “electroplateable” polymers or resins. This is a misnomer in the strict sense, since ABS (and other nonconductive polymers) are incapable of accepting an electrodeposit directly and must be first metallized by other means before being finally coated with an electrodeposit. The conventional technology for electroplating on plastic (etching, chemical reduction, electroplating) has been extensively documented and discussed in the public and commercial literature. See, for example, Saubestre, Transactions of the Institute of Metal Finishing, 1969, Vol. 47., or Arcilesi et al., Products Finishing, March 1984.
Many attempts have been made to simplify the process of electroplating on plastic substrates. Some involve special chemical techniques to produce an electrically conductive film on the surface. Typical examples of this approach are taught by U.S. Pat. No. 3,523,875 to Minklei, U.S. Pat. No. 3,682,786 to Brown et. al., and U.S. Pat. No. 3,619,382 to Lupinski. The electrically conductive film produced was then electroplated. None of these attempts at simplification have achieved any recognizable commercial application.
A number of proposals have been made to make the plastic itself conductive enough to allow it to be electroplated directly thereby avoiding the “electroless plating” process. As noted above, it is common to produce electrically conductive polymers by incorporating conductive or semiconductive fillers into a polymeric binder. Investigators have attempted to produce electrically conductive polymers capable of accepting an electrodeposited metal coating by loading polymers with relatively small conductive particulate fillers such as graphite, carbon black, and silver or nickel powder or flake. Heavy such loadings are sufficient to reduce volume resistivity to a level where electroplating may be considered. However, attempts to make an acceptable electroplateable polymer using the relatively small metal containing fillers alone encounter a number of barriers. First, the most conductive fine metal containing fillers such as silver are relatively expensive. The loadings required to achieve the particle-to-particle proximity to achieve acceptable conductivity increases the cost of the polymer/filler blend dramatically. The metal containing fillers are accompanied by further problems. They tend to cause deterioration of the mechanical properties and processing characteristics of many resins. This significantly limits options in resin selection. All polymer processing is best achieved by formulating resins with processing characteristics specifically tailored to the specific process (injection molding, extrusion, blow molding, printing etc.). A required heavy loading of metal filler severely restricts ability to manipulate processing properties in this way. A further problem is that metal fillers can be abrasive to processing machinery and may require specialized screws, barrels, and the like.
Another major obstacle involved in the electroplating of electrically conductive polymers is a consideration of adhesion between the electrodeposited metal and polymeric substrate (metal/polymer adhesion). In most cases sufficient adhesion is required to prevent metal/polymer separation during extended environmental and use cycles. Despite being electrically conductive, a simple metal-filled polymer offers no assured bonding mechanism to produce adhesion of an electrodeposit since the metal particles may be encapsulated by the resin binder, often resulting in a resin-rich “skin”.
A number of methods to enhance electrodeposit adhesion to electrically conductive polymers have been proposed. For example, etching of the surface prior to plating can be considered. Etching can be achieved by immersion in vigorous solutions such as chromic/sulfuric acid. Alternatively, or in addition, an etchable species can be incorporated into the conductive polymeric compound. The etchable species at exposed surfaces is removed by immersion in an etchant prior to electroplating. Oxidizing surface treatments can also be considered to improve metal/plastic adhesion. These include processes such as flame or plasma treatments or immersion in oxidizing acids. In the case of conductive polymers containing finely divided metal, one can propose achieving direct metal-to-metal adhesion between electrodeposit and filler. However, here the metal particles are generally encapsulated by the resin binder, often resulting in a resin rich “skin”. To overcome this effect, one could propose methods to remove the “skin”, exposing active metal filler to bond to subsequently electrodeposited metal.
Another approach to impart adhesion between conductive resin substrates and electrodeposits is incorporation of an “adhesion promoter” at the surface of the electrically conductive resin substrate. This approach was taught by Chien et al. in U.S. Pat. No. 4,278,510 where maleic anhydride modified propylene polymers were taught as an adhesion promoter. Luch, in U.S. Pat. No. 3,865,699 taught that certain sulfur bearing chemicals could function to improve adhesion of initially electrodeposited Group VIII metals.
For the above reasons, electrically conductive polymers employing metal fillers have not been widely used as bulk substrates for electroplateable articles. Such metal containing polymers have found use as inks or pastes in production of printed circuitry. Revived efforts and advances have been made in the past few years to accomplish electroplating onto printed conductive patterns formed by silver filled inks and pastes.
An additional physical obstacle confronting practical electroplating onto electrically conductive polymers is the initial “bridge” of electrodeposit onto the surface of the electrically conductive polymer. In electrodeposition, the substrate to be plated is often made cathodic through a pressure contact to a metal rack tip, itself under cathodic potential. However, if the contact resistance is excessive or the substrate is insufficiently conductive, the electrodeposit current favors the rack tip to the point where the electrodeposit will not bridge to the substrate.
Moreover, a further problem is encountered even if specialized racking or cathodic contact successfully achieves electrodeposit bridging to the substrate. Many of the electrically conductive polymers have resistivities far higher than those of typical metal substrates. Also, many applications involve electroplating onto a thin (less than 25 micrometer) printed substrate. The conductive polymeric substrate may be relatively limited in the amount of electrodeposition current which it alone can convey. Thus, the conductive polymeric substrate does not cover almost instantly with electrodeposit as is typical with metallic substrates. Except for the most heavily loaded and highly conductive polymer substrates, a large portion of the electrodeposition current must pass back through the previously electrodeposited metal growing laterally over the surface of the conductive plastic substrate. In a fashion similar to the bridging problem discussed above, the electrodeposition current favors the electrodeposited metal and the lateral growth can be extremely slow and erratic. This restricts the size and “growth length” of the substrate conductive pattern, increases plating costs, and can also result in large non-uniformities in electrodeposit integrity and thickness over the pattern.
This lateral growth is dependent on the ability of the substrate to convey current. Thus, the thickness and resistivity of the conductive polymeric substrate can be defining factors in the ability to achieve satisfactory electrodeposit coverage rates. When dealing with selectively electroplated patterns long thin metal traces are often desired. Electrodeposited metal thicknesses of from 1 to 25 micrometer are often typical. The metal traces must normally be of relatively uniform thickness and have a minimum of internal stress. Further, an electrically conductive polymer “seed” pattern defining the traces is often relatively thin, less than about 25 micrometers, and therefore may have relatively low current carrying capacity. These factors of course often work against achieving the desired result.
This coverage rate problem likely can be characterized by a continuum, being dependent on many factors such as the nature of the initially electrodeposited metal, electroplating bath chemistry, the nature of the polymeric binder and the resistivity of the electrically conductive polymeric substrate. As a “rule of thumb”, the instant inventor estimates that coverage rate issue would demand attention if the resistivity of a bulk conductive polymeric substrate rose above about 0.001 ohm-cm. Alternatively, a “rule of thumb” appropriate for thin film substrates would be that attention is appropriate if the substrate film to be plated had a surface “sheet” resistance of greater than about 0.075 ohm per square.
The least expensive (and least conductive) of the readily available conductive fillers for plastics are carbon blacks. Attempts have been made to electroplate electrically conductive polymers using carbon black loadings. Examples of this approach are the teachings of U.S. Pat. Nos. 4,038,042, 3,865,699, and 4,278,510 to Adelman, Luch, and Chien et al. respectively. These earlier efforts were directed primarily at achieving decorative electroplated articles with the substrate fully encapsulated with electrodeposit.
Adelman taught incorporation of conductive carbon black into a polymeric matrix to achieve electrical conductivity required for electroplating. The substrate was pre-etched in chromic/sulfuric acid to achieve adhesion of the subsequently electroplated metal. A fundamental problem remaining unresolved by the Adelman teaching is the relatively high resistivity of carbon loaded polymers. The lowest “microscopic resistivity” generally achievable with carbon black loaded polymers is about 1 ohm-cm. This is about five to six orders of magnitude higher than typical electrodeposited metals such as copper or nickel. Thus, the electrodeposit bridging and coverage rate problems described above remained unresolved by the Adelman teachings.
Luch in U.S. Pat. No. 3,865,699 and Chien et al. in U.S. Pat. No. 4,278,510 also chose carbon black as a filler to provide an electrically conductive surface for the polymeric compounds to be electroplated. The Luch U.S. Pat. No. 3,865,699 and the Chien U.S. Pat. No. 4,278,510 are hereby incorporated in their entirety by this reference. However, these inventors further taught inclusion of materials to increase the rate of metal coverage or the rate of metal deposition on the polymer. These materials can be described herein as “electrodeposit growth rate accelerators” or “electrodeposit coverage rate accelerators”. An electrodeposit coverage rate accelerator is a material functioning to increase the electrodeposition coverage rate over the surface of an electrically conductive polymer independent of any incidental affect it may have on the conductivity of an electrically conductive polymer. In the embodiments, examples and teachings of U.S. Pat. Nos. 3,865,699 and 4,278,510, it was shown that certain sulfur bearing materials, including elemental sulfur, can function as electrodeposit coverage or growth rate accelerators to overcome problems in achieving electrodeposit coverage of electrically conductive polymeric surfaces having relatively high resistivity or thin electrically conductive polymeric substrates having limited current carrying capacity.
In addition to elemental sulfur, sulfur in the form of sulfur donors such as sulfur chloride, 2-mercapto-benzothiazole, N-cyclohexyle-2-benzothiaozole sulfonomide, dibutyl xanthogen disulfide, and tetramethyl thiuram disulfide or combinations of these and sulfur were identified. Those skilled in the art will recognize that these sulfur donors are the materials which have been used or have been proposed for use as vulcanizing agents or accelerators. Since the polymer-based compositions taught by Luch and Chien et al. could be electroplated directly they could be accurately defined as directly electroplateable resins (DER). These directly electroplateable resins (DER) can be generally described as electrically conductive polymers with the inclusion of a growth rate accelerator.
Specifically for the present invention, specification, and claims, directly electroplateable resins, (DER), are characterized by the following features:                (a) presence of an electrically conductive polymer;        (b) presence of an electrodeposit coverage rate accelerator;        (c) presence of the electrically conductive polymer and the electrodeposit coverage rate accelerator in the directly electroplateable composition in cooperative amounts required to achieve direct coverage of the composition with an electrodeposited metal or metal-based alloy.        
In his patents, Luch specifically identified unsaturated elastomers such as natural rubber, polychloroprene, butyl rubber, chlorinated butyl rubber, polybutadiene rubber, acrylonitrile-butadiene rubber, styrene-butadiene rubber etc. as suitable for the matrix polymer of a directly electroplateable resin. Other polymers identified by Luch as useful included polyvinyls, polyolefins, polystyrenes, polyamides, polyesters and polyurethanes.
Using the materials and loadings reported, the carbon black/polymer formulations of Adelman, Luch and Chien referenced above would be expected to have intrinsic “microscopic” resistivities of less than about 1000 ohm-cm.(i.e. 1 ohm-cm., 10 ohm-cm., 100 ohm-cm., 1000 ohm-cm.). When used alone, the minimum workable level of carbon black required to achieve “microscopic” electrical resistivities of less than 1000 ohm-cm. for a polymer/carbon black mix appears to be about 8 weight percent based on the combined weight of polymer plus carbon black. The “microscopic” material resistivity generally is not reduced below about 1 ohm-cm. by using conductive carbon black alone. This is several orders of magnitude larger than typical metal resistivities or resistivities associated with common silver filled inks.
It is understood that in addition to carbon blacks, other well known, highly conductive fillers can be considered to decrease the “microscopic” resistivity of DER compositions. Examples include but are not limited to metallic fillers or flake such as silver. In these cases the more highly conductive fillers can be used to augment or even replace the conductive carbon black. Furthermore, one may consider using intrinsically conductive polymers to supply the required conductivity. For example, an intrinsically conductive polymer in particulate form may be considered as a conductive filler.
The “bulk, macroscopic” resistivity of conductive carbon black filled polymers can be further reduced by augmenting the carbon black filler with additional highly conductive, high aspect ratio fillers such as metal containing fibers. This can be an important consideration in the success of certain applications. Furthermore, one should realize that incorporation of non-conductive fillers may increase the “bulk, macroscopic” resistivity of conductive polymers loaded with finely divided conductive fillers without significantly altering the “microscopic resistivity” of the conductive polymer “matrix” encapsulating the non-conductive filler particles. It has been found that DER formulations can include substantial quantities of non-conductive fillers. In particular, loading of DER formulations with glass fibers has been shown to dramatically reduce mold shrinkage and increase stiffness of these formulations.
Regarding electrodeposit coverage rate accelerators, both Luch and Chien et al. in the above discussed U.S. patents demonstrated that sulfur and other sulfur bearing materials such as sulfur donors and vulcanization accelerators function as electrodeposit coverage rate accelerators when using an initial Group VIII metal electrodeposit “strike” layer. Thus, an electrodeposit coverage rate accelerator need not be electrically conductive, but may be a material that is normally characterized as a non-conductor. The coverage rate accelerator need not appreciably affect the conductivity of the polymeric substrate. As an aid in understanding the function of an electrodeposit coverage rate accelerator the following is offered:                a. A specific conductive polymeric structure is identified as having insufficient current carrying capacity to be directly electroplated in a practical manner.        b. A material is added to the conductive polymeric material forming said structure. Said material addition may have insignificant affect on the current carrying capacity of the structure (i.e. it does not appreciably reduce resistivity or increase thickness). The material need only be present at the substrate surface.        c. Nevertheless, inclusion of said material greatly increases the speed at which an electrodeposited metal laterally covers the electrically conductive surface.It is contemplated that a coverage rate accelerator may be present as an additive, as a species absorbed on a filler surface, or even as a functional group attached to the polymer chain. One or more growth rate accelerators may be present in a directly electroplateable resin (DER) to achieve combined, often synergistic results.        
A hypothetical example might be an extended trace of conductive ink having a dry thickness of 1 micrometer. Such inks typically include a conductive filler such as silver, nickel, copper, conductive carbon etc. The limited thickness of the ink reduces the current carrying capacity of this trace thus preventing direct electroplating in a practical manner. However, inclusion of an appropriate quantity of a coverage rate accelerator may allow the conductive trace to be directly electroplated in a practical manner.
One might expect that other Group 6A elements, such as oxygen, selenium and tellurium, could function in a way similar to sulfur. In addition, other combinations of electrodeposited metals, such as copper and appropriate coverage rate accelerators may be identified. It is important to recognize that such an electrodeposit coverage rate accelerator is important in order to achieve direct electrodeposition in a practical way onto polymeric substrates having low conductivity or very thin electrically conductive polymeric substrates having restricted current carrying ability.
It has also been found that the inclusion of an electrodeposit coverage rate accelerator promotes electrodeposit bridging from a discrete cathodic metal contact to a DER surface. This greatly reduces the bridging problems described above.
Due to multiple performance problems associated with their intended end use, the instant inventor is unaware of any recognizable commercial success for attempts to directly electroplate electrically conductive polymers in applications intended to produce decorative “bright” electroplated objects. Nevertheless, electroplating in a selective manner onto insulating substrates for functional applications remains an intriguing possibility for many applications. This is because electroplating is selective between conductive and insulating surfaces and is inexpensive. Further, a wide variety of metals and alloys can be deposited by electroplating and the deposition rates are relatively rapid. There are a number of techniques available to achieve selective electrodeposited patterns on insulating substrates. Most involve initial formation of a “seed” pattern. The “seed” pattern is formed from a material that has the ability to assist in subsequent metal electrodeposition. Typical “seed” patterns comprise metals, polymers containing electroless plating catalysts, and electrically conductive polymers. Examples of such processes follow in subparagraphs 1 through 3.
(1) An electrically conductive polymer is formed into a “seed” pattern by printing from an ink formulation onto the surface of an insulating substrate. This electrically conductive polymer “seed layer” pattern, when dried, is then subjected to a metal electroplating process to cover the pattern with a conductive metal.
(2) A polymeric composition containing a catalyst suitable for initiating chemical metal deposition is printed into a “seed layer” pattern. After appropriate activation, the article is subjected to a chemical metal deposition “electroless” plating bath. Following coverage with electroless metal, the “seed pattern”, now comprising a layered structure of polymer and chemically deposited metal, is subjected to an electroplating process to cover the pattern with electrodeposited metal.
(3) An insulating substrate is coated in its entirety with a thin film of metal. This uniform coating may be achieved, for example, using vacuum metallizing, sputtering, chemical metal deposition processing. As a next step, a mask is applied having a pattern the reverse of the eventual desired selective metal pattern. The remaining exposed pattern (reverse of the mask pattern) retains its conductive surface and thereby forms a “seed” pattern for subsequent further metal electrodeposition. This subsequent electrodeposition increases metal thickness and also may apply a final coat resistant to an eventual etch. The mask is then removed and the article etched to completely remove the metal that had been covered by the mask.
As previously noted, the current inventor is unaware of any recognizable success in attempts to use DER technology to produce decorative “bright” electroplated objects. Nevertheless, the current inventor has persisted in personal efforts to overcome certain performance deficiencies associated with the initial DER technology. Along with these efforts has come a recognition of unique and eminently suitable applications employing the DER technology for functional applications. These functional applications often have requirements, such as selectivity, fine patterning and relatively thin electrodeposits, which differ substantially from the requirements of purely decorative electroplating. Some examples of these unique applications for electroplated articles include solar cell electrical current collection grids and interconnect structures, electrodes, electrical circuits, electrical traces, circuit boards, antennas, capacitors, induction heaters, connectors, switches, and resistors. One readily recognizes that the demand for such functional applications for electroplated articles is relatively recent and has been particularly explosive during the past decade.
It is important to recognize a number of important characteristics of directly electroplateable resins (DER's) which may facilitate certain embodiments of the current invention. One such characteristic of the DER technology is its ability to employ polymer resins and formulations generally chosen in recognition of the fabrication process envisioned and the intended end use requirements. In order to provide clarity, examples of some such fabrication processes are presented immediately below in subparagraphs 1 through 9.                (1) Should it be desired to electroplate an ink, paint, coating, or paste which may be printed or formed on a substrate, a good film forming polymer, for example a soluble resin such as an elastomer, can be chosen to fabricate a DER ink (paint, coating, paste etc.). For example, in some embodiments thermoplastic elastomers having an olefin base, a urethane base, a block copolymer base or a random copolymer base may be appropriate. In some embodiments the coating may comprise a water based latex. Other embodiments may employ more rigid film forming polymers. The DER ink composition can be tailored for a specific process such flexographic printing, rotary silk screening, gravure printing, ink jet printing, flow coating, spraying etc. Furthermore, additives such as tackifiers or curatives can be employed to improve the adhesion of the DER ink to various substrates.        (2) Very thin DER traces often associated with collector grid structures can be printed and then electroplated due to the inclusion of the electrodeposit growth rate accelerator. Traces as thin as 1.5 micrometer have been demonstrated as practical. Silk screening has produced trace widths as little as 150 micrometers.        (3) Should it be desired to cure the DER substrate to a 3 dimensional matrix, an unsaturated elastomer or other “curable” resin may be chosen.        (4) DER inks can be formulated to form electrical traces on a variety of flexible substrates. For example, should it be desired to form electrical structure on a laminating film, a DER ink adherent to the sealing surface of the laminating film can be effectively electroplated with metal and subsequently laminated to a separate surface.        (5) Should it be desired to electroplate a fabric, a DER ink can be used to coat all or selected portion of the fabric intended to be electroplated. Furthermore, since DER's can be fabricated out of the thermoplastic materials commonly used to create fabrics, the fabric itself could completely or partially comprise a DER. This would eliminate the need to coat the fabric.        (6) Should one desire to electroplate a thermoformed article or structure, DER's would represent an eminently suitable material choice. DER's can be easily formulated using olefinic materials which are often a preferred material for the thermoforming process. Furthermore, DER's can be easily and inexpensively extruded into the sheet like structure necessary for the thermoforming process.        (7) Should one desire to electroplate an extruded article or structure, for example a sheet or film, DER's can be formulated to possess the necessary melt strength advantageous for the extrusion process.        (8) Should one desire to injection mold an article or structure having thin walls, broad surface areas etc. a DER composition comprising a high flow polymer can be chosen.        (9) Should one desire to vary adhesion between an electrodeposited DER structure supported by a substrate the DER material can be formulated to supply the required adhesive characteristics to the substrate. For example, the polymer chosen to fabricate a DER ink can be chosen to cooperate with an “ink adhesion promoting” surface treatment such as a material primer or corona treatment. In this regard, it has been observed that it may be advantageous to limit such adhesion promoting treatments to a single side of the substrate. Treatment of both sides of the substrate in a roll to roll process may adversely affect the surface of the DER material and may lead to deterioration in plateability. For example, it has been observed that primers on both sides of a roll of PET film have adversely affected plateability of DER inks printed on the PET. It is believed that this is due to primer being transferred to the surface of the DER ink when the PET is rolled up.        
All polymer fabrication processes require specific resin processing characteristics for success. The ability to “custom formulate” DER's to comply with these changing processing and end use requirements while still allowing facile, quality electroplating is unique among methods to electroplate onto polymeric forms.
Another important recognition regarding the suitability of DER's for the teachings of the current invention is the simplicity of the electroplating process. Unlike many conventional electroplated plastics, DER's do not require a significant number of process steps prior to actual electroplating. This allows for simplified manufacturing and improved process control. It also reduces the risk of cross contamination such as solution dragout from one process bath being transported to another process bath. The simplified manufacturing process will also result in reduced manufacturing costs.
Yet another recognition of the benefit of DER's for the teachings of the current invention is the ability they offer to selectively electroplate metal onto an article or structure. The desired metal structures of the invention often involve long yet fine metal traces. Further, the articles of the invention often consist of such metal patterns selectively positioned in conjunction with flexible insulating materials. Such selective positioning of metals can often be expensive and difficult. As discussed previously, the coverage rate accelerators included in DER formulations allow for such extended surfaces to be covered with electrodeposit in a relatively rapid and simple manner.
Yet another recognition of the benefit of DER's is their ability they to be continuously electroplated. As will be shown in later embodiments, it is often desired to continuously electroplate metal onto “seed” patterns defining specific structure. DER's are eminently suitable as “seed” patterns for such continuous electroplating.
Yet another recognition of the benefit of DER's for the teachings of the current invention is their ability to withstand the pre-treatments often required to prepare other materials for plating. For example, were a DER to be combined with a metal, the DER material would be resistant to many of the pre-treatments such as cleaning which may be necessary to electroplate the metal.
Another important recognition is the suitability of metal electrodeposition for producing articles of the current invention. Electroplating is a rapid and inexpensive metal deposition process. Electroplating allows selective deposition of a wide variety of metals and alloys. Single or multi-layered deposits may be chosen for specific attributes. Examples may include copper for conductivity and nickel, silver or gold for corrosion resistance. Electrodeposition further allows a wide range of appropriate deposit thickness to be achieved relatively quickly. The articles of the invention may require metal traces varying from about 0.1 micrometer to greater than about 100 micrometer (i.e. 0.1 micrometer, 1 micrometer, 10 micrometer 25 micrometer, 100 micrometer etc.). Such thicknesses may be readily achieved in reasonable time using metal electrodeposition.
These and other attributes of DER's and electroplating may contribute to successful articles and processing of the instant invention. However, it is emphasized that the DER technology, and more broadly electroplating onto conductive polymeric “seed” layers, is but one of a number of alternative metal deposition or positioning processes suitable to produce many of the embodiments of the instant invention. Other approaches, such as electroless metal deposition, selective metal etching, placement of metal forms such as wires or strips, stamping metal patterns and selective vacuum or chemical deposition may be suitable alternatives for producing the selectively positioned metal structures of the invention. These choices will become clear to the skilled artisan in light of the teachings to follow in the remaining specification, accompanying figures and claims.
Another important aspect of the embodiments of the current invention is the inclusion of web processing to achieve structure and combinations in a facile and economic fashion. A web is a generally planar or sheet-like structure, normally flexible, having thickness much smaller than its length or width. This sheet-like structure may also have a length far greater than its width. A web of extended length may be conveyed through one or more processing steps in a way that can be described as “continuous”, thereby achieving the advantages of continuous processing. “Continuous” web processing is well known in the paper and packaging industries. It is normally accomplished by supplying web material from a feed roll of extended length to the process steps. The product resulting from the process is often continuously retrieved onto a takeup roll following processing, in which case the process may be termed roll-to-roll or reel-to-reel processing.
An advantage of web processing is that the web can comprise many different materials, surface characteristics and forms. The web may comprise layers for packaging material options such as pressure sensitive or hot melt adhesive layers, environmental barriers, and as support for printing and other features. The web can constitute a nonporous film or may be a fabric and may be transparent or opaque. Combinations of such differences over the expansive surface of the web can be achieved. Indeed, as will be shown, the web itself can comprise materials such as conductive polymers or even metal fibers which will allow the web itself to perform electrical function. The web material may remain as part of the final article of manufacture or may be removed after processing, in which case it would serve as a surrogate or temporary support during processing.
In order to eliminate ambiguity in terminology, for the present invention the following definitions are supplied:
While not precisely definable, for the purposes of this specification, electrically insulating materials may generally be characterized as having electrical resistivities greater than 10,000 ohm-cm. Also, electrically conductive materials may generally be characterized as having electrical resistivities less than 0.001 ohm-cm. Also electrically resistive or semi-conductive materials may generally be characterized as having electrical resistivities in the range of about 0.01 ohm-cm to about 10,000 ohm-cm. The characterization “electrically conductive polymer” covers a very wide range of intrinsic resistivities depending on the filler, the filler loading and the methods of manufacture of the filler/polymer blend. Resistivities for electrically conductive polymers may be as low as 0.00001 ohm-cm. for very heavily filled silver inks, yet may be as high as 10,000 ohm-cm or even more for lightly filled carbon black materials or other “anti-static” materials. “Electrically conductive polymer” has become a broad industry term to characterize all such materials. Thus, the term “electrically conductive polymer” as used in the art and in this specification and claims extends to materials of a very wide range of resistivities from about 0.00001 ohm-cm. to about 10,000 ohm-cm and higher.
An “electroplateable material” is a material having suitable attributes that allow it to be coated with a layer of electrodeposited material.
A “metallizable material” is a material suitable to be coated with a metal deposited by any one or more of the available metallizing process, including chemical deposition, vacuum metallizing, sputtering, metal spraying, sintering and electrodeposition.
“Metal-based” refers to a material or structure having at least one metallic property and comprising one or more components at least one of which is a metal or metal-containing alloy.
“Alloy” refers to a substance composed of two or more intimately mixed materials.
“Group VIII metal-based” refers to a substance containing by weight 50% to 100% metal from Group VIII of the Periodic Table of Elements.
A “bulk metal foil” refers to a thin structure of metal or metal-based material that may maintain its integrity absent a supporting structure. Generally, metal films of thickness greater than about 2 micrometers may have this characteristic. Thus, in most cases a “bulk metal foil” will have a thickness between about 2 micrometers and 250 micrometers (i.e. 2 micrometer, 5 micrometer, 10 micrometer, 25 micrometer, 100 micrometer, 250 micrometer) and may comprise a laminate of multiple layers.
The term “monolithic” or “monolithic structure” is used in this specification and claims as is common in industry to describe an object that is made or formed into or from a single item or material.
The term “continuous form” refers to a material structure having one dimension of sufficient size such that it can be fed to or retrieved from a process over an extended time period without interruption of the material structure.
The term “continuous process” refers to a process or method in which at least one of the material feed components or a product of the process has a continuous form.
A web is a generally planar or sheet-like structure, normally flexible, having thickness much smaller than its length or width.
“Web processing” is a process wherein a web itself is altered by the process or wherein structure supported by the web is added, altered or otherwise modified by the process.
A “reel to reel” or “roll to roll” process is one wherein at least one of the feed components to a process is supplied in a continuous roll form and the product of the process is retrieved on a takeup roll.